An absolute monarchy in a post Age of Enlightenment human civilization makes no sense.
I wonder.
What difference is there between an absolute monarch and a dictator with absolute power who passes it to his son? A dictator with absolute power has certainly occurred in a post Age of Enlightenment world. What does it matter what the dictator calls himself?
All government comes down to is power. Mao said that political power grows out of the barrel of a gun. In Charted Space political power grows out of warships in orbit. No one believes that the Emperor has a divine right to rule. They believe he has the military power to rule.
The worlds are like separate little boxes. Communication and movement between the boxes takes weeks, months, or even years. Events on one don't affect the others. Things would have to be so bad in each little box of 10,000 boxes all at the same time for long enough that all the boxes risk the wrath of the Imperial Navy to revolt. All else ends up in a game of revolt whack-a-mole. Only the Solomani got away with a successful revolt, and that took a relatively high level of ethnic and cultural unity and a centuries-long military buildup.
It's something like how relatively few guards control a far greater number of prisoners. The prisoners are all separated in cells. In Charted Space, the worlds and the realities of interstellar travel and communication are the cells, and the guards are the Imperial Navy. This invalidates many of the assumptions relied on when thinking about this sort of thing.
Populations don't have the power to enforce their will on anything outside their worlds. A billion people can't do anything outside their world if they don't have the fleet power to both protect their star system and project military force. They can riot as much as they want, they'll only be hurting themselves. They can form a citizen army with its fist raised to the sky, and then sit there. Worlds are by nature very vulnerable to defeat in detail, or simply being bypassed and ignored. They can't form a gigantic interstellar mob and march on Capital to guillotine the Emperor. They'd need a fleet more powerful than the Imperial Navy.
Planetary spokesman: "We raise a flag of Freedom! Here's to Liberty, the Rights of Humaniti, and Interstellar Revolution!
Imperial naval commander: "Execute firing solution. Open fire."
In Charted Space, only one world experienced the Age of Enlightenment, Terra. The Solomani spread the ideals of the Age of Enlightenment far and wide, but how well were those ideas received by worlds which never had anything like them in their cultural tradition? After the Long Night, did worlds outside of the Solomani Sphere even remember them? Perhaps like democracy in Athens, they lasted for a time, then were swept away.